The IB-peptide marker in the V-region of mouse immunoglobulin light chains is genetically linked to the Ly-3.1 thymocyte surface antigen. The inbred strains of mice bearing these genetic markers demonstrate a high incidence of spontaneous leukemia. We are studying the relationships between these characteristics, and are investigating the structures of IB-positive immunoglobulin and the Ly-antigens. The 13-day foetal mouse thymus is being studied in organ culture to characterize the steps involved in the differentiation and maturation of thymic lymphocytes. Emphasis is placed upon the role of the thymic epithelium in this process. The foetal mouse thymus in organ culture will also be injected with murine RNA leukemia viruses in an attempt to study under culture conditions the leukemogenic potential of these agents. BIBLIOGRAPHIC REFERENCES: Durda, P.J. and Gottlieb, P.D. The Ly-3 Antigens on Mouse Thymocytes: Immune Precipitation and Molecular Weight Characterization, J. Exp. Med. 144:476 (1976). Gottlieb, P.D. and Durda, P.J. The IB-Peptide Marker and the Ly-3 Surface Alloantigen: Structural Studies of a VK-region Polymorphism and a T-Cell Marker Determined by Linked Genes, Cold Spring Harbor Symposium on Quantitative Biology, XLI, 1976, in press.